1. Field of the Invention
The present invention relates generally to a lithium secondary battery provided with a positive electrode, a negative electrode, and a non-aqueous electrolyte prepared by dissolving a solute in a non-aqueous solvent, and is characterized in that deterioration of a variety of characteristics is suppressed ensuring a sufficient battery capacity and a sufficient battery voltage even under high temperature conditions.
2. Description of the Related Art
Recently, as one of advanced secondary batteries featuring high power and high energy density, lithium secondary batteries of high electromotive force, using a non-aqueous electrolyte prepared by dissolving a solute in a non-aqueous solvent and utilizing oxidation and reduction of lithium have come into practical use.
In such lithium secondary batteries, lithium-manganese composite oxide having a spinel structure, lithium-cobalt composite oxide and lithium-nickel composite oxide having a layer structure and so on are generally used as a positive electrode active material of a positive electrode.
In the lithium secondary battery, when the lithium-manganese composite oxide having the spinel structure is used as the positive electrode active material, battery voltage is higher compared with a case where the lithium-cobalt composit oxide having the layer structure is used, however, there have remained problems that battery capacity is small and preservation characteristics at high temperature conditions are degraded.
On the other hand, in the lithium secondary battery using the lithium-nickel composite oxide having the layer structure as the positive electrode active material, the battery capacity is large, but the battery voltage is low and the preservation characteristics at high temperature are not improved sufficiently.
Therefore, in recent years, there has been proposed to use a mixture of the lithium-manganese composite oxide having the spinel structure and the lithium-nickel composite oxide having the layer structure as the positive electrode active material of the positive electrode, to appropriately set a balance between the battery capacity and the battery voltage and to improve the preservation characteristics at high temperature (see, the script collection of the 43rd battery discussion, pp. 162-163 and the script collection of the 44th battery discussion, pp. 308-309).
Unfortunately, however, the lithium secondary battery using such a mixture of the lithium-manganese composite oxide having the spinel structure and the lithium-nickel composite oxide having the layer structure as the positive electrode active material has not fully suppressed the deterioration of battery characteristics such as charge/discharge characteristics in storage under high temperature conditions.
Further, in such a lithium secondary battery, the non-aqueous electrolyte wherein a lithium salt, such as LiPF6 or LiBF4 is dissolved in the non-aqueous solvent such as ethylene carbonate, propylene carbonate or dimethyl carbonate is used.
Unfortunately, however, such a lithium secondary battery using the non-aqueous electrolyte wherein the lithium salt, such as LiPF6 or LiBF4 is dissolved in the non-aqueous solvent, such as ethylene carbonate, propylene carbonate or dimethyl carbonate has problems that the battery capacity is decreased and a battery internal resistance is increased in the storage under the high temperature conditions.
Therefore, in recent years, for the purpose of suppressing an increase in the battery internal resistance in the storage, there has been proposed to use the non-aqueous electrolyte wherein a main solvent is the ethylene carbonate and vinylene carbonate is admixed in the range of 0.01 to 10.0 wt % based on the ethylene carbonate (see, Japanese Patent No. 3066126).
Unfortunately, however, even using the non-aqueous electrolyte wherein the vinylene carbonate is admixed in the aforesaid range based on the ethylene carbonate, the increase in the battery internal resistance has not been fully suppressed in the storage under the high temperature conditions.
Additionally, in recent years, there has been proposed to use lithium bis(oxalato)borate as the solute in the non-aqueous electrolyte in order to improve cycle performance of the lithium secondary battery under the high temperature conditions (see, Kang Xu, Shengshui Zhang, Bruce A. Poese, and T. Richard Jow; “Lithium Bis(oxalato)borate Stabilizes Graphite Anode in Propylene Carbonate”; Electrochemical and Solid-State Letters, 5(11)A259-A262(2002)).
Unfortunately, however, even in a case where the lithium bis(oxalato)borate is used as the solute in the non-aqueous electrolyte of the lithium secondary battery, there has remained the problems that the battery internal resistance is increased and the battery characteristics such as the charge/discharge characteristics are deteriorated.